Translation (Growth and Development 2012)

7/31/2019 Translation (Growth and Development 2012)

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Atif

H.

Khirelsied,

Ph.D.

DepartmentofBiochemistry

Facult ofMedicine

InternationalUniversity

of

Africa


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Requirementsofproteinsynthesis

Ribosomes

mRNA

tRNA Aminoacids

AminoacyltRNAsynthetaseenzyme

Stagesofproteinsynthesis:Translation

2. Elongation

3. Termination4. Posttranslationalprocessing

n orso rans a on an o cs


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structure.

Theinformationrequiredforthesynthesisofaproteinis

storedintheformofanucleotidebasesequenceintheDNA.

Thisnucleotidebasesequence,calledthecistronformsa

.


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,

transcribedfromthecorrespondinggeneintheDNA,inthe

.

ThemRNAgoestotheribosomeswherethemessageis

translatedintotheprotein.

ProteinTranscription mRNA Translation


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r osome, epro e nsyn es z ngmac nery.

mRNA,whichcarriestheinformationdetermining

.

tRNA,carriesmatchingaminoacidstotheribosome.

Aminoacids,buildingunitsofpolypeptides.


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AminoacyltRNAsynthetaseenzyme,bindsamino

acidstothespecificcarriertRNA.

, .

Proteinfactors,whicharerequiredforthe

interactionoftheribosomewithmRNAandtRNA(initiation,elongation andterminationfactors).


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RibosomesarelargecomplexesofproteinandrRNA.

Theyconsistoftwosubunitsonelargeandonesmall

whoserelativesizesaregenerallygivenintermsoftheir

sedimentationcoefficients,orS(Svedberg)units.

Theprokaryotic50Sand30Sribosomalsubunitstogether

formaribosomewithanSvalueof70.

ribosome.


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EukaryoticribosomeProkaroytic ribosome

80S70S

60S

40S

50S

30S

5.8S

RNA

5S5SRNA

28S 18S16S23S

~30Proteins~50

Proteins32

Proteins 21

Proteins


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Ribosomesmayexistfreeinthecytosol orboundto

therou hendo lasmicreticulum.

Freeribosomessynthesizecytosolic proteins.

Roughendoplasmicreticulumribosomessynthesize

membraneboundandextracellularproteins.


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A,P,andEsites. Thesesitescoverthreeneighboringcodons.


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nontranslated leadingsequence,atranslatedcoding

,

usuallyendsinapoly(A)tail.

Initiating

codon

Terminating

codon

Nontranslated

5 3

Nontranslated Translatedcodingsequence


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Thereisatleastones ecifictRNA s eciesforeach

aminoacid.

SomeaminoacidshavemorethanonetRNA species.

In rokar otesthereareabout30tRNA s ecies while

ineukaryotesthereareabout50tRNA species.

Thereare22tRNA speciesinmitochondria.


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Anticodons recognizethecomplementarycodons inthe

mRNAinantiparallelfashion.

Ifthecodonis5AUG3,theanticodon thatrecognizesitis

.

baseintheanticodon,bothreadinthe5to3direction.

ThemostimportantdifferencebetweenthevarioustRNAspeciesofthesameaminoacidliesinthe3rd nucleotidesin

the anticodon.


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Alltwent standardaminoacidsarere uiredfor

proteinsynthesis.

Ifonlyoneoftheseaminoacidsismissing,translation

stops

at

the

codon

which

codes

for

this

amino

acid.

Thisexplainswhythedeficiencyofonlyoneessential

biologicalvalue.


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t

Inthecytosol,thereare20speciesofthisenzyme,one

specificforeachofthe20aminoacidsrequiredfor

proteinsynthesis.

acidwiththe3OHofthespecifictRNA.

TheenzymerecognizestheRradicaloftheaminoacid

andtheDarmofthetRNA.


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AminoacyltRNA

Aminoacid

ATPsynt etase

PPi Activationoftheaminoacid

AMP Aminoacid

CCA

occursintwostepscatalayzed

by

the

same

enzyme:

AMP1. TheaminoacidreactswithATP,formin enz mebound

aminoacylAMP.

CCA Aminoacid2. Theenzymetransferstheaminoacidtothespecific

tRNA formin aminoac ltRNA.

AminoacyltRNA


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Theprocessoftranslationisdividedintothreestages

1. Initiation

. onga on

3. Termination

Each sta e com rises several ste s.


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Initiationofproteinsynthesisinvolvestheassemblyofthe

componentsofthetranslationmachinerywhichinclude:

1. thetworibosomalsubunits,

. ,

3. theaminoacyltRNA specifiedbythefirstcodoninthe

4. Energy(GTPandATP)

5. initiationfactors.


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Initiation ofproteinsynthesisoccursinfoursteps:

1. Dissociationoftheintactribosomeintoitscomponent

subunits.

. .

3. Formationofthe48Sinitiationcomplex.

4. Formationofthe80Sinitiationcomplex


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.

subunits.


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Formationofthe43S

preinitiation complex

Formationofthe48S


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Formationofthe80S

initiation

complex


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ShineDalgarno sequence:

. , ,

knownastheShineDalgarno sequence,islocatednear

its5end. Thissequenceisrecognizedbya

complementaryoneinthe16SrRNA.


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Specific initiator tRNA :

initiatortRNA thatenterstheribosomalPsite.

ThisrecognitionisfacilitatedbyIF2inE.coli andseveral

.


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Ineukaryotes,the40Sribosomalsubunitbindstothe

downthemRNAuntilitencounterstheinitiatorAUG

codonwithinaconsensusKozak sequence

oza sequence


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Elongationofproteinrequires:

.

2. theaminoacyltRNA specifiedbythe2ndcodon

inthemRNA.

. nergy an .

4. elon ation factors.


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Elongation

occurs

in

three

steps:1. Incorporationofthe2nd aminoacyttRNA attheA

.

2. Peptidebondformation(transpeptidation)

3. translocation


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A.BindingofAminoacyltRNAattheASite

BindingoftheaminoacyltRNA totheAsite

requirestheactionofanelongationfactor(EFTu

an s npro aryo esan neu aryo es

andtheh drol sisofGTPtoGDPandPi.


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B.FormationofaPeptideBond

e reeam nogroup n e am noacy

formsapeptidebondwiththecarbonylofthe1st

am noacy a e s e.

Formationofthepeptidebondiscatalyzedbythe

peptidyl transferase activity

in

the

large

ribosomal

subunit.


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B.FormationofaPeptideBond

e a e s enow oesno con a nan

aminoacid.Itisuncharged,andthepolypeptideis

a a c e o e n e s e.


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.

Afterthe e tidebond formation theribosome

moves threenucleotidestowardthe3endofthe

mRNA.

,

G and

the

hydrolysis

of

GTP

in

E.

coli

(EF

2,in

eukaryotes).


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.

This displacestheunchargedtRNA intothe

ribosomalEsite(exitsite)andmovesthepeptidyl

t ntot e s te.

Theelongationandtranslocationstepsarerepeated

untilaterminationcodon movesintotheAsite.


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Entryofthe2nd aminoacyl

tRNA in

the

A

site

of

the

80Scomplex


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Peptidebondformationcatalyzed

bypeptidyl transferase

Translocation


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ThereisnotRNA torecognizingstopcodon(UAA,UAG,

UGA).

ReleasingfactorRF1recognizesthatastopcodonresides

ormsacomp exw re eas ng ac or an .

Thiscomplex,withthepeptidyl transferase,causes

hydrolysisofthebondbetweenthepeptideandthetRNA

occupyingthePsite.


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Review


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.

subunits.


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Formationofthe43S

preinitiation complex

Formationofthe48S


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Formationofthe80S

initiationcomplex


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Entryofthe2nd aminoacyl

tRNA intheAsiteofthe80S


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Peptidebondformationcatalyzed

bypeptidyl transferase

Translocation


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Isabacteriostatic thateffectiveagainstongrampositive

bacteria.

n stot e r osoma su un to acter a, n t ng

translocationofpeptidyltRNA.

Erythromycin


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subunitofbacteria

InhibitsbindingofaminoacyltRNA totheAsite.


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ofthe50Sribosomalsubunitofbacteria.

Highdosesinhibitmitochondrialribosomalenzyme.


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Isstructura ysimi artotyrosy tRNA.

n sa e s e, ormsa

peptidebondwiththegrowing

peptidechain,causingpremature

terminationofproteinsynthesis

bothinprokaryotesandin

.


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Thecomparativestructuresofthe

antibiotic

puromycin (top)and

the

3terminalportionoftyrosinyltRNA (bottom).


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Isanin i itoro protein iosynt esisineu aryotes

ro uce y e ac er um rep omycesgr seus.

Cycloheximide


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Dip t eriatoxin astwosu units,t eBsu unit in stoa

cellsurfacereceptorandfacilitatestheentryoftheA

su un n o ece .

inhibitionofelongationfactor2(EF2).


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.

Insulinistranslatedasaproteincontaining109aminoacids

knownaspreproinsulin.


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.

Aphosphatemaybeesterified tothealcoholgroupof

serine,threonine,ortyrosine.

Thisisver im ortantintheactivationinactivationof

manyenzymes.


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Glycosylation

Sugarradicalsmaybeconnectedtothealcoholgroupof

serine threonine or l sine 0linked or to the amide

groupofasparagine (Nlinked)toformglycoproteins.


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Acetylation

Acetylradicalsmaybeconnectedtothe aminogroupof

lysineresidueinproteins(histones ).


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H drox lation

Prolyl andlysyl residuesarehydroxylated incollagen.This

isver im ortantforh dro enbondin andformationofstrongcollagen.


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Glutamateresiduesiscarboxylated insomeproteins,

. ., , , .

proteinstobindcalciumtoperformtheirfunction.


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Carboxylase enzymescontainbiotinlinkedtothe

.


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Manyproteinscontainaprostheticgrouplinkedto

Translation (Growth and Development 2012)

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